The present invention relates to a mechanically controlled brake power booster, in particular for brake systems of automotive vehicles, comprising a dish-shaped working piston movably arranged in a housing and connected to a substantially cylindrically shaped control casing accommodating a valve device to control the working pressure in a chamber having a control piston connected to an actuating rod which is held in a direction opposite to the actuating direction by a holding member radially inserted into the control casing and secured in its mounting position.
In a known vacuum brake booster of the foregoing type, for example, German Patent DE-AS No. 1,293,028, which serves to reinforce the brake pressure in an automotive vehicle, the working piston and control casing are integrally formed. The control piston carried in the control casing is held in a direction opposite to the actuating direction by a holding device radially inserted into a slot of the control casing. The slot opens into an annular groove holding the inner sealing bead of a rolling diaphragm. In the area above the slot, the holding member forms the bottom of the groove so that it is held in its mounted position by the sealing bead. Arranging the holding member in this manner has the disadvantage of not always ensuring a reliable seal in the area of the radially outer end surface of the holding member. Therefore, on actuation of the power booster, atmospheric air may enter through the leak from the working chamber into the vacuum chamber and impair the operation of the power booster.
Further, it is known, for example, from German Patent DE-OS No. 2,208,762, to manufacture the working piston and the control casing of a vacuum brake booster as separate components and to connect them in a pressure-tight manner. In this arrangement, the working piston is placed into seating engagement with a surface of the control casing and in the actuating direction it is in abutment with an annular collar of the control casing. In this mounted position, the working piston is held in position by the inner sealing bead of a rolling diaphragm engaging into a groove in the control casing. This construction has the disadvantage that the connection of the working piston is exclusively maintained by the sealing bead of the rolling diaphragm which may cause sealing bead wear and leakage.